1. Field of the Invention
The present invention relates generally to a method of cleaning a wafer with a cleaning solution, and which method allows the solution to be used for a long duration, thereby preventing waste and contributing to the efficiency of the cleaning process as a whole. More particularly, the present invention relates to a method of cleaning a wafer with a standard cleaning 1 solution, in which steps, such as adding NH.sub.4 OH and H.sub.2 O.sub.2, or NH.sub.4 OH, H.sub.2 O.sub.2 and H.sub.2 O to the solution, are carried out to maintain the composition of the solution, whereby the solution does not have to be replaced as often.
2. Description of the Related Art
Semiconductor integrated circuit devices are fabricated on and in a semiconductor wafer by processing the wafer through many processing stations. The wafer may be contaminated by impurities while it is transported between processing stations and/or during each process carried out at one of such stations. These impurities are generally removed from the wafer by a so-called wafer cleaning process. All of the semiconductor wafers are subjected to an RCA cleaning process in which the wafer is first cleansed with a high temperature organic solvent before being processed, and is again cleansed after each process of the fabrication method is carried out.
The RCA cleaning process for the silicon wafers involves the use of two kinds of cleaning solutions. First, the standard cleaning 1 (SC1) solution which comprises 1:1:5 to 1:2:7 of NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O is used. The SC1 solution oxidizes organic contaminants on the surface of the silicon wafer under the action of the H.sub.2 O.sub.2 contained therein. Organic contaminants may be present because of the incomplete removal of a photoresist, and also because of airborne materials and the physical handling of the wafer. The NH.sub.4 OH contained in the SC1 solution effectively removes heavy metal contaminants such as cadmium, cobalt, copper, iron, mercury, nickel or silver by forming amino complexes with them. When the amount of NH.sub.4 OH in the SC1 becomes low, the surface roughness of the wafer will be decreased. Accordingly, the SC1 solution can reduce to having 1:2:10 or 1:4:20 by volume of NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O.
Subsequently, the standard cleaning 2 (SC2) solution having a composition of 1:1:6 to 1:2:8 by volume of HCl:H.sub.2 O.sub.2 :H.sub.2 O is used to remove alkaline metals such as aluminum or magnesium. The SC2 may be used alone or as mixed with a diluted HF solution.
Although the SC1 solution is effective in removing the contaminants on the silicon wafer surface, the time-dependent variation of the NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O composition is so great that it can only be used for a short time. See, e.g., Isao Takahashi et al. "Time-Dependent Variation of Composition of SC1 Solution", Jpn. J. Appl. Phys. Vol. 32 (September 1993), pp L1183-L1185.
FIG. 1, which is taken from the above-identified reference, is a graph showing the variation of NH.sub.4 OH concentration depending on the time and temperature of the SC1 solution. In FIG. l, the abscissa indicates time in minutes and the ordinate indicates the NH.sub.4 OH concentration in molar concentration per unit volume (mol/l). The point .box-solid. indicates the initial condition, the line--.circle-solid.--indicates the NH.sub.4 OH concentration at 21.degree. C., the line--.box-solid.--at 50.degree. C., and the line--.tangle-solidup.--at 80.degree. C. When the initial composition of the SC1 solution was NH.sub.4 OH: H.sub.2 O.sub.2 : H.sub.2 O=1:1:5, the NH.sub.4 OH concentration decreased with time, as shown in FIG. 1. The decrease in the rate was dependent on the temperature, i.e., the decrease in rate accelerated with increases in temperature. For example, for the temperature of 80.degree. C., the NH.sub.4 OH concentration decreased to 1/10 of its original value after 100 minutes.
The experiments performed by the present inventors to evaluate the time-dependent variation of the composition of the SC1 solution (NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O=1:4:20) showed a decrease in rate of 0.07% by weight of NH.sub.3 /min and an increase in rate of 0.0037% by weight of H.sub.2 O.sub.2 /min, as shown in FIG. 2. In FIG. 2, the abscissa indicates time in minutes, the left ordinate indicates H.sub.2 O.sub.2 concentration in % by weight, and the right ordinate indicates NH.sub.3 concentration in % by weight.
The decrease in the concentration of NH.sub.4 OH, which exists in the form of NH.sub.3 molecules in the SC1 solution, results from the spontaneous evaporation of the NH.sub.3. In particular, since the wafer cleaning process using the SC1 solution is carried out at an elevated temperature, the life of the solution is very short.
Generally, the wafer cleaning process is carried out as a batch process. One batch indicates a run of the process in which a certain number (e.g., 50) of wafers are placed in a cleaning bath containing the SC1 solution and the cleaning process is performed at a predetermined temperature. When the SC1 solution having NH.sub.4 OH H.sub.2 O.sub.2 :H.sub.2 O=1:4:20 by volume is used, the solution should be replaced with a fresh one after 5 (five) batches are run or after about 100 minutes of use.
Such a frequent replacement of SC1 solution yields large volumes of waste solutions and is an obstacle in reducing the time necessary to complete the cleaning process.